(1) Field of the Invention
The present invention relates to a system for driving a laser diode used as a light source in a laser beam printer or the like, and more particularly to a system for stopping emission from the laser diode in action.
(2) Description of the Prior Art
With a laser beam printer, it is necessary to stop laser emission when the printer is switched on and a video signal is input, for safety's sake and to avoid unnecessary laser emission outside an image forming time.
For this purpose, a known laser beam printer employs an emission stopping circuit system as shown in FIG. 1. This system comprises a laser diode 31, a differential amplifier 32, and a DC supply circuit 33. The differential amplifier 32 and DC supply circuit 33 constitute a laser diode drive circuitry 34. The DC supply circuit 33 supplies direct current above the threshold level to the laser diode 31.
The differential amplifier 32 receives a video signal at one input terminal 32a and a feedback signal at the other input terminal 32b. The video signal is applied to the input terminal 32a of the differential amplifier 32 after undergoing level adjustment through a DC bias voltage circuit 35 for assuring that the laser diode 31 is used only in a laser emission range (above "Do" in FIG. 4). The feedback signal is produced by detecting emission from the laser diode 31 with a PIN photodiode 36 and amplifying the resulting signal with an output monitor circuit 37 comprising a video buffer amplifier. The output monitor circuit 37 amplifies the feedback signal to a level corresponding to one to one relationship with the video signal. Consequently, the feedback signal applied to the input terminal 32b of the differential amplifier 32 causes the laser diode 31 to increase and decrease laser emission following the level of the video signal.
The circuit system further comprises an ON/OFF circuit 38 including a photocoupler 39 and an output transistor 40 for stopping the laser emission. The photocoupler 39 has a light emitting diode 39a for receiving a signal from a CPU, not shown, for controlling the emission from the laser diode 31.
When forward current is applied to the light emitting diode 39a, the diode 39a emits light whereby a phototransistor 39b becomes conductive. With the operation of phototransistor 39b, the base voltage of the transistor 40 drops to zero volts and the transistor 40 becomes nonconductive, whereby the video signal is applied to the input terminal 32a of the differential amplifier 32 in a normal way. When forward current is not applied to the light emitting diode 39a, the diode 39a stops emitting light whereby the phototransistor 39b becomes nonconductive. As a result, the transistor 40 becomes conductive and the input terminal of the differential amplifier 32 is applied to ground through the collector and emitter of transistor 40, whereby the laser diode 31 stopes the emission.
With the above known systems, it is difficult to cause the laser diode 31 to stop the laser emission completely since the laser diode 31 is controlled through the output transistor 40 of the ON/OFF circuit 38. More particularly, even when the output transistor 40 is conductive, its internal resistance maintains the voltage between the collector and the emitter at several millivolts instead of allowing the voltage to fall to zero. Since the collector of the output transistor 40 is connected to the input terminal 32a of the differential amplifier 32 through a point P2, the several millivolts are applied to the differential amplifier 32 as a leakage input. As a result, an output from the differential amplifier 32 is applied to the laser diode 31 even when the video signal is zero. This leakage input causes the laser diode 31 to emit light since the diode 31 is operable by the DC supply circuit 33 even with a slight voltage applied thereto through the differential amplifier 32. The amount of emission in this instance is dependent on the level of leakage input, namely the performance and resistance of the output transistor 40. The amount of emission is also influenced by the maximum input level of the video signal controlled by the DC bias voltage circuit 35. That is, while the video signal level at a point P1 varies from 0V to 2V by reason of the system construction, the maximum level at point P2 may be reduced to half of the level at point P1 or less in order to secure responsivity of the laser diode drive circuitry 34. In such a case, the level of the leakage input applied through the output transistor 40 rises relative to the video signal level at point P2, thereby increasing the amount of leakage emission.
When the above laser diode drive system is applied to a printer, the leakage emission does not always give rise to a problem. This is true where, for example, a sensitive material irradiated by a laser beam to form an image thereon has low sensitivity, or where the sensitive material has high sensitivity but is exposed outside an image forming area to the leakage emission. With the laser beam printer, however, scanning often is repeated numerous times along one line on the sensitive material; the first scan for recording an image and subsequent scans without the laser emission. If leakage emission takes place during the second and subsequent scans, the image area of the sensitive material becomes exposed thereby damaging image quality.